It is known that several fields of application are open for polyurethanes which can be prepared from polyols and polyisocyanates, but the reaction of the starting components generally proceeds so rapidly even at room temperature that the reaction mixtures (two-component systems) are quite unstable in storage even at room temperature. If one-component systems which are stable in storage are to be obtained, it is therefore necessary to eliminate the reactivity of the isocyanate groups and/or hydroxyl groups by reversible blocking.
One-component systems of this kind include, for example, the known mixtures of polyhydroxyl compounds and polyisocyanates having blocked isocyanate groups (see e.g. Polyurethanes, Chemistry and Technology, Part I, Interscience Publishers, 1962, page 8). Compounds containing blocked groups which are normally reactive with isocyanate groups are, for example, the oxazolidines described in German Offenlegungsschrift No. 2,446,438. Mixtures of polyisocyanates and these oxazolidines are stable in storage at room temperature and it is only when they are exposed to water (atmospheric moisture) that they release hydroxyl and amino groups and are converted into systems which react to form polyurethane ureas. The preparation of these urethane oxazolidines is carried out in a multistage reaction via the intermediate stage of the corresponding N-hydroxyalkyloxazolidines which are reacted with polyisocyanates to yield urethane oxazolidines. If desired, the polyisocyanates used in this reaction may be prepolymers with free isocyanate groups based on the usual polyhydroxyl compounds used in polyurethane chemistry. These prepolymers are known per se.
It is an object of the present invention to provide a simple process for reversibly blocking the usual polyhydroxyl compounds used in polyurethane chemistry. This problem could be solved by the invention described below. The present invention does not only provide a new method for the preparation of polyhydroxyl compounds with blocked hydroxyl groups but also permits to simultaneously reduce the viscosity of the starting polyhydroxyl compounds so that the products of the invention unless they are branched often exhibit a considerably lower viscosity than the starting polyhydroxyl compounds.